This invention relates to a method and apparatus for controlling charging speeds of an injection molding machine.
Even when the number of revolutions of the screw of an injection molding machine is maintained at a constant value and even when the back pressure at the time of retracting the screw is maintained at a constant value, the quantity of resin accumulated in a space in front of the screw, the length to diameter ratio of the screw and the state of melting the resin vary from time to time due to a small change in particle diameter of a resin to be injected, a manufacturing lot, etc., thus changing the molten state of the resin. Consequently, the feed speed caused by the rotation of the screw varies more or less which results in the variation in the retracting speed of the screw. According to a prior art solution proposed to solve this problem, the retracting speed of the screw is detected by a detector, the detected speed is compared with a reference retraction speed preset in a setter and the number of revolutions of the screw is controlled so as to eliminate the difference between the detected speed and the reference speed thereby controlling the charging speed.
The resin plasticized in a barrel is melted by the heat transmitted from heating means provided for the barrel, and the shearing force imparted by the screw rotation and the amount of heat given to the resin are proportional to the stay time of the resin in the screw groove. However, the stay time of the resin injected in one lot is not always constant. Moreover, as the depth of the screw groove is not constant throughout the length of the screw, apparent heat conductivity varies from point to point thus rendering nonuniform the resin temperature. Moreover, the shearing force caused by the screw rotation also causes nonuniform the resin temperature because the shearing force becomes small as the degree of plasticization increases while the screw is retracted during which the effective length (a length between a resin hopper and the front end of the screw) decreases. Thus, according to the method described above wherein the resin charging speed is controlled by varying the number of revolutions of the screw, the shearing force or energy imparted to the resin varies. Consequently, all the molten resin in the screw groove is influenced by the variation in the shearing force thus failing to obtain uniform temperature and uniform kneading.
The purpose of control utilizing the screw back pressure is to vary the pressure in the screw groove at the front end thereof so as to improve the kneading effect caused by the flow of the resin in the groove due to back pressure. The inventor has found the fact that the back pressure has an influence on a limited quantity of the resin contained in the groove between the front end of the screw and a point a predetermined distance apart therefrom and that the charging speed can be efficiently controlled by controlling the back pressure.